This invention relates to a heater for an indirect-heated cathode which is difficult to disconnect and is very reliable.
Conventional heaters for indirect-heated cathodes which have been generally used are manufactured through the following process.
(1) A tungsten wire (or a wire containing tungsten as the main component) (12) is wound around the molybdenum (or alloy containing molybdenum as the main component) wire (11) at a predetermined pitch as shown in FIG. 1a.
(2) After finishing the above process, the above two wires are formed into the shape shown in FIG. 1b.
(3) After forming the shape in the above step (2), alumina having the appropriate particle size is coated on the wires through for example electrodeposition.
(4) Under a reducing atmosphere in which tungsten cannot be oxidized, the alumina coating is sintered for example at a temperature of 1650 degrees C. to strengthen the alumina coating.
(5) The molybdenum core wire is dissolved and removed in strong acid (for example, mixed solution of hydrochloric acid and nitric acid) to thereby obtain a heater with the appearance shown in FIG. 1c. The schematic cross section of A--A line in FIG. 1c is shown in FIG. 1d. In these drawings, (11) is a molybdenum wire, (12) is tungsten wire, (33) is alumina covering, and (34) is the hollow portion.
Since the heating portion can be concentrated near the point to be heated in the indirect-heated cathode in the heater thus manufactured, an indirect-heated cathode with good electron emission can be obtained with comparatively low electric power.
On the other hand, the portion which has been left after dissolving and removing the molybdenum wire as shown in FIG. 1d has become hollow and the mechanical strength of the heater is decreased.
If the heater is used for a cathode ray tube, a short-circuit current melts the tungsten wire when flashover occurs in the tube. Also, the difference in thermal expansion coefficient exerts repeated tensile stress on the tungsten wire from alumina, causing the tungsten wire to disconnect. These problems occur in conventional heaters.